1. Field of the Invention
The present invention relates to techniques of a semiconductor device provided with a photodiode, a manufacturing method thereof, and an optical disc device.
2. Description of Related Art
In an optical disc device, a photodetector IC that receives light reflected from an optical disc and converts the light to an electric signal has been used. The photodetector IC is a semiconductor device including a photodiode, which is a light receiving element, and a semiconductor integrated circuit, such as a bipolar integrated circuit and a MOS (Metal Oxide Semiconductor) integrated circuit formed of transistors and the like, all of which are formed on the same substrate.
In the semiconductor device including a photodiode and a semiconductor integrated circuit as above, incident light is converted to a current by the photodiode and the current is further converted to a voltage, and a predetermined processing is applied to the voltage to output a signal.
FIG. 13 is a schematic cross section of a semiconductor device 100 in the related art including a photodiode and a semiconductor integrated circuit. The semiconductor device 100 includes a photodiode 101 and a circuit region having an NPN-type bipolar transistor 102 and so forth.
To be more concrete, the semiconductor device 100 includes a P-type semiconductor substrate 110 and a heavily doped P-type semiconductor layer 111 is formed thereon, on which is further formed a lightly doped P-type epitaxial layer 112, which is a semiconductor layer having a lower impurity concentration than the heavily doped P-type semiconductor layer 111. Further, an N-type epitaxial layer 113 and a heavily doped N-type diffusion layer 114 are sequentially formed on the lightly doped P-type epitaxial layer 112 in a region of the photodiode 101. The heavily doped N-type diffusion layer 114 will form a charge extraction region to extract charges generated in the photodiode 101. It also plays a role of enhancing a frequency characteristic by lowering resistance of the cathode region in the photodiode 101. In addition, a heavily doped P-type diffusion layer 115 is formed adjacently to the heavily doped N-type diffusion layer 114 on the lightly doped P-type epitaxial layer 112. A lightly doped P-type diffusion layer 116 is formed adjacently beneath the heavily doped P-type diffusion layer 115.
According to the structure in the related art as above, because not only the heavily doped N-type diffusion layer 114 but also the N-type epitaxial layer 113 is a cathode region in the region of the photodiode 101, an electric field is not applied to the vicinity of the PN junction sufficiently. Accordingly, a depletion layer on the bottom surface side does not extend sufficiently, which makes a parasitic capacitance larger. In addition, because the heavily doped N-type diffusion layer 114 in the cathode region and the heavily doped P-type diffusion layer 115 that will form an anode extraction region are joined directly, a parasitic capacitance is large.
Consequently, a parasitic capacitance of the entire photodiode 101 becomes larger, which makes it difficult to make the photodiode 101 faster.
To eliminate such an inconvenience, there has been proposed a semiconductor device that reduces a parasitic capacitance by extending the depletion layer in the anode region by changing an N-type epitaxial layer in the photodiode region to a lightly doped P-type semiconductor layer by means of ion implantation as described, for example, in JP-A-2007-317767. Also, there has been proposed a semiconductor device in which a photodiode is formed directly on a P-type semiconductor layer as described, for example, in JP-A-2006-210494.